Why not just go buy one? They aren’t that expensive.
The build parties are most popular in California where every gun sale must go through a dealer and be recorded by the state. I imagine the same restrictions apply to other leftist hell holes. Then you have those who just don’t want to leave a paper trail. Even in Texas when I see people selling AR’s person to person the seller usually insists on doing a transfer even though it is not legally required.
To your point I have found that I can get a good lower shipped and transfered for less than the cost of an 80% lower.
Because there is no record or background check with a 80% lower.